1. Field of the Invention
This invention relates to a driving control method for an image pickup device, an image pickup apparatus, and an image pickup system applied to a camera unit used mainly for factory automation (FA), for example, for picking up an image of an object moving at a high speed.
This application claims priority of Japanese Patent Application No. 2002-133529, filed on May 9, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
As an image pickup device used for an image pickup apparatus such as a camera unit, a field storage-type solid-state image sensor such as a CCD image sensor is used. In some image pickup apparatuses using such a solid-state image sensor, the charge storage time of the solid-state image sensor is controlled, thereby adjusting exposure without using any mechanical iris mechanism and enabling high-speed shutter.
For example, in an image pickup apparatus used for FA, an external trigger is generated at the timing of detection output of an object sensor which detects an object transported by a belt conveyor or the like. Since a high speed and a constant speed of exposure are demanded of such an image pickup apparatus for industrial applications, the image pickup apparatus controls the charge storage time of the solid-state image sensor on the basis of this external trigger, thereby adjusting the exposure time to pick up an image of the object.
Specifically, in the image pickup apparatus, a shutter pulse SUB is generated at the timing of the leading edge of an external trigger pulse TRIG, as shown in FIG. 1. In response to this shutter pulse SUB, electric charges stored in the solid-state image sensor up to this point are swept away to a substrate and storage of electric charges is newly started. After that, a read-out pulse (sensor gate pulse) SG is generated. In response to this read-out pulse SG, the stored electric charges are read out. Therefore, the time from the generation of the shutter pulse SUB until the generation of the read-out pulse SG is the time when electric charges are stored. This time t1 is exposure time.
In this case, the shutter pulse SUB and the read-out pulse SG are usually controlled to be generated within a blanking period t2 of a video signal VIDEO so as not to affect an output image.
However, in the case of picking up an image with a slow shutter (long exposure time), as the timing of the external trigger pulse TRIG needs to be made early, it is set within an output period t3 of an image signal based on the trigger pulse TRI that is immediately before, as shown in FIG. 2. Since electric charges stored in the solid-state image sensor during the output period t3 of the image signal are swept away, also the image signal based on the electric charges is swept away, causing a problem that noise 20 is generated in an image displayed on a display device 10 such as a monitor device, as shown in FIG. 3.
Meanwhile, in the image pickup apparatus as described above, in order pickup an image of an object in response to the demand for reduction in inspection time of the object, increase in speed of the belt conveyor or the like and shortening of the cycle of the external trigger pulse TRIG are required.
However, when the external trigger pulse TRIG has a short cycle, the shutter pulse SUB generated by the external trigger pulse TRIG has a short cycle, too. In this case, too, the timing of the external trigger pulse TRIG is set within the output period t3 of the image signal based on the trigger pulse TRIG that is immediately before, as shown in FIG. 2. Therefore, there arises the problem of generation of noise 20 in the image displayed on the display device 10, as shown in FIG. 3.
Thus, when an image is picked up with a particularly slow shutter (long exposure time) and the cycle of the external trigger pulse TRIG is shortened, the possibility of generation of noise 20 in the image increases.
In this case, generation of noise 20 may be prevented if the external trigger pulse TRIG is controlled to be generated within the blanking period t2. To do this, however, it is necessary to judge the narrow blanking period t3 from the timing of the synchronizing signal of the image sent out to the image pickup apparatus and generate the external trigger pulse within this period t3. The control for this is not easy and the structure of the device for generating the external trigger is complicated. Moreover, when the image pickup apparatus operates in accordance with an internal synchronizing signal instead of the synchronizing signal of the image sent from outside, the synchronizing signal of the image of the image pickup apparatus must be outputted to an external device and the narrow blanking period must be recognized in the external device. In this case, too, the structure of the device is complicated.
These problems are not limited to the image pickup apparatus for industrial applications. In typical image pickup apparatuses which perform shutter control at the timing of a trigger pulse TRIG, similar problems arise when the cycle of the external trigger pulse TRIG is shortened or when the exposure time is elongated.